Dr. Abbas Khan, 32, an orthopedic surgeon from Streatham in south London, went to Aleppo in November 2012 to help civilian victims of the ongoing civil war, the BBC reported.
Syrian security officials promised Khan's family he would be released this week, but notified the family Monday he had died.
"My brother was going to be released at the end of the week. We were given assurance by the Syrian government," said Khan's brother, Afroze Khan. "My brother knew that. He was ready to come back home. He was happy and looking forward to being released."
"We are devastated, distraught and we are angry at the Foreign Office for dragging their feet for 13 months," he added.
A spokesman for the British Foreign Office said they were "concerned by reports that a British national has died in detention in Syria."
"We are urgently seeking clarification of this from the Syrian authorities," the spokesman, whose name wasn't reported, said.